The present invention pertains to a gas-measuring system with an open optical measuring path for the spectroscopic measurement of at least one component of a gas sample with a laser source, a reference gas sample for the gas to be measured, two radiation detectors for the main beam and the reference beam and at least two radiation reflectors.
A prior-art gas-measuring system of this type with an open optical measuring path (so-called open-path measurement) has been known from DE 196 11 290 C2 (and U.S. Pat. No. 5,767,976), wherein the transmitting and receiving optical systems are located close to one another in space and a retroreflector is additionally used. One essential drawback of this prior-art gas-measuring system is due to the interference with and the attenuation of the measured signal due to the beam passing through the necessary optical elements beam splitter and retroreflector. The optical path measurement of gases is, in general, the detection of trace amounts of gaseous substances which may be present at extremely low concentrations. The prerequisite for the optical quantification is, however, the presence of absorption bands of the gases to be detected in a spectral range accessible for the optical measuring technique being used. To reach a low detection limit, a spectral range is expediently selected in which the gas to be analyzed has a pronounced infrared activity, i.e., intense optical absorption, and in which the lowest possible cross sensitivities, especially with atmospheric gases such as water or carbon dioxide, prevail. An intense optical absorption by gas molecules usually takes place in the spectral range of the principal molecular vibrations, which are often in one of the two wavelength ranges of 2 to 5 micrometer (xcexcm) and 8 to 12 micrometer (xcexcm). On the other hand, the prior-art optical arrangements and radiation sources are characterized by relatively poor received measured signal quality, which is due especially to the radiation sources used in combination with the optical elements used.
The object of the present invention is to provide an improved gas-measuring system of the type mentioned in the introduction with an open optical measuring path without a retroreflector, which does not make it necessary to use a beam splitter.
According to the invention, a gas-measuring system is provided with an open optical measuring path for the spectroscopic measurement of at least one component of a gas sample with a laser source, a reference gas sample for the gas to be measured, two radiation detectors for the main beam and the reference beam and at least two radiation reflectors. The laser source is provided to emit a divergent ray beam, from which both the measuring beam and the reference beam are formed after a single-time reflection of the emitted ray beam. A first radiation reflector is provided in the form of a first concave mirror for the reflection of a first part of the ray beam emitted from the laser source as a measuring beam into the open optical measuring path. A second radiation reflector is provided in the form of a second concave mirror for the reflection of a second part of the ray beam emitted from the laser source as a reference beam into a reference gas cuvette containing the reference gas sample for the gas to be measured. A third radiation reflector is provided in the form of a third concave mirror for the reflection of the measuring beam received after passing through the optical measuring path onto a first radiation detector.
The first radiation reflector may be a paraboloidal mirror designed as an asymmetric mirror in relation to the optical axis. The second radiation reflector may be designed as a spherical mirror or a paraboloidal mirror.
The laser source may be a near infrared laser diode or a quantum cascade laser. The optical measuring path may advantageously be 1 to 200 meters (m).
According to a further aspect of the invention, a process is provided including using the system as described above for detecting one or more of the gases hydrogen sulfide (H2S), ammonia (NH3), hydrochloric acid (HCl), phosgene (COCl2), carbon monoxide (CO), and methane (CH4).
One essential advantage of the present invention arises from the use of very few optical elements, so that disturbances and loss of intensity of the measuring beam are extensively avoided. The object is accomplished especially by a first radiation reflector being designed in the form of a concave mirror and especially preferably in the form of a paraboloidal mirror which is asymmetric in relation to the optical axis, a so-called off-axis paraboloidal mirror, so that the divergent ray beam emitted by the laser source sweeps both this first radiation reflector and a second radiation reflector arranged in the vicinity, which is in the form of a concave mirror and preferably in the form of a spherical mirror or paraboloidal mirror, in a specific manner. The ray beam emitted by the first radiation reflector forms the measuring beam, which reaches a first radiation detector after passing through the measuring path via a third radiation reflector. The ray beam reflected by the second radiation reflector is used as a reference beam and is directed toward a second radiation detector via a reference gas cuvette containing a reference gas sample of the gas to be measured.
An especially preferred laser source is a quantum cascade laser, which emits a pulsating radiation and with which the optical wavelength range of 2 to 12 micrometers (xcexcm) of the near to middle infrared range, which is of particular interest, can be covered.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawing and descriptive matter in which preferred embodiments of the invention are illustrated.